docs: def fill_subparser(subparser):
      """Sets up a subparser to download the CIFAR-10 dataset file.
  
      The CIFAR-10 dataset file is downloaded from Alex Krizhevsky's
      website [ALEX].
  
      Parameters
      ----------
      subparser : :class:`argparse.ArgumentParser`
          Subparser handling the `cifar10` command.
  
      """
      url = 'http://www.cs.toronto.edu/~kriz/cifar-10-python.tar.gz'
      filename = 'cifar-10-python.tar.gz'
      subparser.set_defaults(urls=[url], filenames=[filename])
      return default_downloader